When drilling with a drilling tool according to German patent No. 27 30 418, which corresponds to U.S. Pat. No. 4,230,429, in solid metal material, on withdrawal of the drill from the workpiece a waste disc is formed, whose diameter corresponds to the hole diameter. This waste disc is caused by the fact that the carbide tips are arranged at the same height in the axial direction of the drill. This arrangement was selected so that for a given material thickness or a given drilling depth the drill has to cover as short as possible a drilling distance. When making blind holes too, the aim is for the radially outer tip if possible to cut to exactly the same depth as the radially inner tip. But if several stacked or superimposed plates which are not welded together in the region of drilling are drilled with a drilling tool of this kind, then one of the waste discs described above would be formed on passage of the drill through each individual plate. It has been shown that these discs can pass between the superimposed plates, which can lead not only to the plates being forced apart, but also to damage to the drilling tool.
For this reason, special drilling tools were developed for drilling in plate stacks of the kind mentioned hereinbefore. In a known drilling tool of this kind (catalogue of "KOMET KUB reversible-tip drills" of the firm Komet Stahlhalter- und Werkzeugfabrik Robert Breuning GmbH, D-7122 Besigheim, KUB 10/83, page 44), the radially outer, second reversible carbide tip is axially set back from the first reversible carbide tip, and also their two operative cutting edges form acute angles of different size with the drill axis. Due to this design, during axial movement of the drill the material is continuously removed from the inside to the outside, and thus formation of the above-mentioned waste discs on withdrawal of the drill from the plate and the drawbacks arising with them are avoided. But as the two operative cutting edges of the outer reversible carbide tip form acute angles in the same direction with the drill axis in this special drilling tool, which is also called a "plate drill," these two cutting edges produce an uncompensated radial cutting force component which forces the drill in a direction towards the drill axis, as seen from the outer carbide tip. As a result, the hole becomes slightly smaller than the nominal diameter of the drill.
It is the object of the invention to provide a drilling tool for drilling in solid metal material, in particular for drilling in plate stacks, of the kind mentioned hereinbefore, in which the radial cutting force components are compensated as far as possible, so that lateral displacement of the drill is avoided and, in particular in plate stacks too, precision holes of accurate dimensions can be drilled while avoiding waste discs.
According to the invention, this is achieved by the fact that the first reversible carbide tip is smaller than the second reversible carbide tip, and on the same radius as the first reversible carbide tip is arranged a third reversible carbide tip which is also smaller than the second reversible carbide tip, in such a way that its operative cutting edge works approximately on the same working cone and on the same maximum radius as the radially outer cutting edge of the second reversible carbide tip.
Due to the use of three reversible carbide tips, two of which are smaller than the third, by varying the cutting width and also by varying the angles which the operative cutting edges form with the drill axis, compensation of the radial cutting force components to a large extent can be obtained. The radially outer cutting edge of the third reversible carbide tip largely compensates the radial cutting force component occurring at the outer operative cutting edge of the second reversible carbide tip. Moreover, especially in the last stage shortly before complete withdrawal from a plate, the drilling tool is guided only by the two outer cutting edges of the second and third reversible carbide tips. As the two cutting edges are arranged at the same angle to the drill axis and also work on the same working diameter, radial cutting force components are produced which are of equal quantity but in opposite directions and which center the drilling tool. This ensures that precision holes of accurate dimensions are made. The new drilling tool is suitable not only for drilling in plate stacks, but also for drilling in one-piece solid metal material.